Regenerative thermal oxidizers (RTOs) are used in a number of industries to reduce the quantity of contaminants in process effluent gases. RTOs are unique in their ability to conserve fuel through the use of heat exchangers. In an RTO, the process effluent gases are oxidized in a combustion chamber. As the high-temperature combustion gases move to an exhaust stack, they flow through a heat exchanger, typically a chamber containing ceramic media. In the heat exchanger, up to 95% of the heat is transferred from the gases to the ceramic media. The flow of gases is then reversed such that the inlet process gases move through the heat exchanger toward the combustion chamber. Heat is transferred from the hot ceramic media to the process gases and consequently less energy is required to oxidize the process gases in the combustion chamber.
RTOs are continually subjected to plugging by organic and inorganic particulates. Organic particulates, such as heavy hydrocarbons and cellulose particulates can be readily disposed of with a bake-out feature in a RTO. This bake-out cycle elevates the temperature of the recovery chambers to a point where the organic particulates carbonize and fall away from the heat recovery media. These carbonized particulates are then carried out with the stack exhaust gas.
On the other hand, this is not the case with inorganic particulate such as silica which lodges in the oxidizer. Solid silica coming in as part of the process air stream is not a problem; it passes through the RTO media like fine beach sand. The real problem exists when silicon fume enters the RTO. In passing through the regenerative heat recovery bed, silicon fume is preheated to 95% of the units combustion chamber oxidation temperature, which is usually greater than 1400° F. When silicon laden fume passes through the point in the bed where the temperature is greater than 1292° F. (700° C.), it is oxidized and becomes a solid particulate on the surface of the ceramic media. It is believed that the interface boundary always remains above the original media surface where the original free silicon dioxide resides. In other words, the silica layering builds from the inside out by forming consecutive layers on the original host silica such that new layers are formed under the previously formed layers.
Eventually, depending on the amount of silicon fume present in the process gas, a RTO will become fouled to the point where process flow is choked off completely. Although SiO2 oxidation builds up slower as the RTO oxidation process continues, unfortunately by the time it gets to the nil formation point, most RTOs are incapacitated by insufficient process flow.
Typically, most end users that have the problem of silica build up on ceramic media enter the RTO periodically and try to clean off the silica that has adhered to the ceramic heat recovery media with the use of a high pressure water stream or with the use of compressed air. Some even try to remove the top layers of the media where the silica has adhered, wash it and then replace it. There are as many methods of cleaning and/or maximizing the duration time between replacements of media as there are users with the silica build up problem.